Large cast support structures have traditionally not been used in applications where fail-safe performance of the structure is required. One example of such an application would be in connection with the manufacture of large commercial aircraft. In such instances, large support structures, and particularly large support structures such as engine pylons, have been constructed with a large plurality of independent parts and fastening elements designed so that the failure of any one of the constituent parts does not result in failure of the entire structure. It has generally been believed that a monolithic structural support element, formed from a casting process, is not ideal for use as a primary component of an aircraft because of heretofore recognized inherent limitations with large, cast structures. More particularly, with previously developed large, cast structures, when such structures develop a crack, the crack may propagate through the structure in response to repeated cyclic loading on the structure. Thus, a failure beginning in one element of a large, cast support structure can eventually result in the failure of the entire structure.
Further limitations with large, cast structures involve the weight typically associated with such structures. Until the present time, the manufacture of a monolithic cast structural component has generally been made using steel or aluminum. In aircraft applications, where weight is of paramount importance, casting a large truss-like support structure from steel would yield a component that is simply unacceptably heavy.
Some large structural parts have been cast from aluminum, but aluminum also has drawbacks when attempting to apply it to components to be used with aircraft structures. Most notably, aluminum is not suitable for areas of an aircraft where the component will experience high heat, such as an engine pylon, which experiences significant heat generated by the jet engine.
The use of a large, cast structure as a primary structure on an aircraft would also significantly simplify the construction of modern day commercial aircraft. For example, an engine strut (i.e., pylon) which is used on an aircraft to attach an engine to a wing of the aircraft is typically assembled from a large plurality of independent, complex parts. For commercial aircraft, a typical engine strut is usually composed of a hundred or more independent parts, including various shims, which are held together by thousands of fasteners. The connections require extensive drilling of holes, corrosion protection and sealing of joints and fasteners. The cost of just the engine strut is a significant portion of the total cost of manufacturing an aircraft despite the structure being a relatively small fraction of the aircraft's total mass. Any manufacturing improvement that reduces the overall cost of such an assembly while maintaining a redundant structure that assures against failure, despite the structure being located in an environment of high loading, high sonic fatigue, high temperature, and corrosive gases, would be very desirable. Other examples of such applications might be the carry-through structure used to connect the right and left horizontal tail planes into a single structural assembly, supports for the landing gear, or attachment of fuel tanks or other external equipment to the aircraft (to name a few potential applications).
Accordingly, there is still a need for producing primary structural components for aircraft and other structures that are structurally strong and resistant to structural failure, yet which are lighter than present day structural assemblies. There is a particular need for such structural components that are also capable of handling the high loading, high fatigue, and highly corrosive environments experienced by various structures used in commercial aircraft applications.